Methylhydroxyalkylcellulose is generally prepared by alkalization of pulp and subsequent reaction with etherifying agents such as methyl chloride, ethylene oxide or propylene oxide in the presence of suitable solvents or suspension media (cf. Ullmanns Enzyklopädie der technischen Chemie, 4th Edition, Volume 9, page 205, or Houben-Weyl, Methoden der organischen Chemie, 4th Edition, Volume E 20, part 3, page 2047). The crude methylhydroxyalkylcelluloses obtained in the preparation of methylhydroxyalkylcellulose contain undesirable by-products such as sodium chloride, methanol, glycols, etc. Depending on the degree of substitution, the methylhydroxyalkylcellulose prepared displays a differing, temperature-dependent dissolution behaviour in aqueous media. The methylhydroxyalkylcelluloses described here are generally insoluble in hot water and flocculate out, so that the undesirable by-products are, according to the prior art, dissolved by suspension of the crude methylhydroxyalkylcellulose in hot water and are washed out of the crude methylhydroxyalkylcellulose in a subsequent separation and washing step. When the term methylhydroxyalkylcellulose suspension is used in the following, it is, unless indicated otherwise, always a suspension of crude methylhydroxyalkylcellulose and the by-products obtained in the preparation of crude methylhydroxyalkylcellulose in hot water. Some of the separation, purification and washing methods used according to the prior art are described in EP 03 05 898 A2, EP 03 05 899 A2 and EP 0 632 056 B1. Depending on the field of application of the methylhydroxyalkylcellulose, it has to meet more or less strict purity requirements, so that a suitable washing step has to be an indispensable part of the preparative process.
To be able to prepare products for a wide variety of applications, it has to be possible to prepare methylhydroxyalkylcelluloses having different degrees of substitution. The alkyl substitution is generally described by the DS in cellulose ether chemistry. The DS is the mean number of substituted OH groups per anhydroglucose unit. The methyl substitution is reported, for example, as DS (methyl) or DS (M). The hydroxyalkyl substitution is usually described by the MS. The MS is the mean number of moles of etherifying reagent bound via an ether bond per mole of anhydroglucose unit. The etherification with the etherifying reagent ethylene oxide is reported, for example, as MS (hydroxyethyl) or MS (HE). The etherification with the etherifying reagent propylene oxide is correspondingly reported as MS (hydroxypropyl) or MS (HP). The determination of the side groups is carried out by the Zeisel method (reference: G. Bartelmus and R. Ketterer, Z. Anal. Chem. 286 (1977) 161-190).
The degree of substitution of the methylhydroxyalkylcellulose has an influence on the required temperatures of the hot water suspension and the washing liquids used in the washing step. The temperatures of the hot water suspension and the washing liquids are usually in the range from 80° C. to 100° C., preferably from 85° C. to 98° C.
Various properties of the methylhydroxyalkylcellulose, e.g. the thermal flocculation point, the solubility in water or organic solvents, the viscosity of the solutions produced, the film formation capability, the water retention capability or the adhesive strength, are set via the degree of etherification and the type of substituents.
Owing to these many excellent properties, methylhydroxyalkylcelluloses are used in a wide variety of applications.
Firstly, they are used in industrial applications as, for example, thickeners, adhesives, binders, dispersants, water retention agents, protective colloids, stabilizers, suspension media, emulsifiers, film formation agents and as consistency regulators and processing aids in mineral and dispersion-based building materials systems.
Secondly, they are used in sensitive applications, for example in the production of cosmetic and pharmaceutical preparations, in the coating of tablets, in eye drop suspensions, in contact lens cleaners and the like or in the production of foodstuffs in which cellulose ethers having a particularly high purity are required (cf. WO 00/32637). High purity means adherence to the appropriate pharmaceutical standards (e.g. pharmaceutical monographs), for example in respect of degree of substitution, heavy metal content, viscosity of the solution, organic impurities, etc. Furthermore, further legal regulations and additional wide-ranging customer requirements have to be met.
Thirdly, methylhydroxyalkylcelluloses are used in high-value industrial applications, for example in the production of speciality ceramics and in suspension polymerization, which likewise require a particularly high purity. In these applications, high purity means, in particular, low contamination of a physical type.
An important quality criterion for customers in the sensitive applications and high-value industrial applications is visual assessment of the methylhydroxyalkylcellulose in respect of physical contamination, known as coloured particles.
Neither pharmaceutical nor other regulations gives specifications in respect of coloured particles in methylhydroxyalkylcellulose for sensitive applications and high-value industrial applications. The assessment of this quality criterion has to be undertaken by customers and is generally carried out by means of subjective, visual assessment. Accordingly, the assessment of methylhydroxyalkylcellulose in respect of coloured particles is carried out by comparison with samples which are assessed after provision of samples by the customer.
The following four classifications are obtained:                (1) very good, i.e. better than a comparative sample provided by the customer        (2) good, comparable with a comparative sample provided by the customer        (3) acceptable, only slightly worse than a comparative sample provided by the customer and        (4) not acceptable, i.e. worse than a comparative sample provided by the customer.        
EP 1 375 523 A1 describes a method of selecting methylhydroxyalkylcellulose in order to obtain, firstly, improved properties in the filtration of aqueous solutions of the methylhydroxyalkylcellulose and, secondly, to obtain films having no defects from the aqueous solution of the methylhydroxyalkylcellulose. The method described here is in principle also suitable for preparing methylhydroxyalkylcellulose having a low content of coloured particles. However, a disadvantage of the method described in EP 1 375 523 A1 is obviously that selection in industrial production processes always leads to the inevitable formation of amounts of product which do not meet the selection criteria and accordingly have to be disposed of, subjected to a reprocessing step or processed further in another way, which costs money.
After the crude methylhydroxyalkylcellulose has been suspended in hot water, the methylhydroxyalkylcellulose is separated off from the suspension medium by methods of the prior art, generally by pressing, centrifugation or filtration, with a distinction being made between discontinuously and continuously operating separation techniques (Grundoperationen chemischer Verfahrenstechnik, 11th Edition, page 164).
In principle, continuously and discontinuously operating pressing apparatuses, e.g. multiplaten presses, strainer presses, plate filter presses, travelling screen presses and screw presses, are suitable for separating off the methylhydroxyalkylcellulose from the suspension medium. Centrifuges are likewise suitable for separating off methylhydroxycellulose from the suspension medium. Known types of centrifuge include, for example, turnout centrifuges, peeler centrifuges, pusher centrifuges, mesh screw centrifuges, vibrating centrifuges and sliding centrifuges and decanters.
A further method which has attained importance for separating off methylhydroxyalkylcellulose is filtration. Here, a distinction is likewise made between discontinuous and continuous apparatuses. Discontinuously operating filters include, for example, fixed-bed filters, suction filters, candle filters, leaf filters and plate filters. The separation of methylhydroxyalkylcellulose from the suspension medium by means of discontinuously operating filters is generally less preferred. A disadvantage here is the loading and unloading of the filter, which requires a considerable time. Thus, discontinuously operating filters are not suitable for sufficiently large, economical throughputs.
Continuously operating filters such as belt filters and rotary filters have been found to be most useful as separation apparatuses, and rotary pressure filters as are known from WO 02/100512 A1 are particularly suitable.
These rotary pressure filters allow methylhydroxyalkylcelluloses which are used, for example, in industrial applications to be separated off economically on an industrial scale. However, if the methylhydroxyalkylcellulose is to be used in sensitive applications or high-value industrial applications which require a high purity, then the conventional rotary pressure filters optimized for the industrial applications and the corresponding materials which come into contact with the suspension and thus with the methylhydroxyalkylcellulose are not suitable.
It has been found that the use of conventional rotary pressure filters in the preparation of methylhydroxyalkylcellulose for sensitive applications and high-value industrial applications leads to an unacceptable number of coloured particles in the methylhydroxyalkylcellulose. The colour of the particles can, depending on the rotary pressure filter used, vary from greyish through brownish through to black colours.
One considerable way in which these coloured particles can be formed is associated with the unavoidable abrasion of the plastics used. This abraded material leads to no impairment of the product in the preparation of methylhydroxyalkylcellulose for industrial applications.
In the production process for methylhydroxyalkylcellulose for sensitive applications and high-value industrial applications, the suspensions of the crude methylhydroxyalkylcellulose and the associated by-products are obtained in particular compositions which display an increased degree of abrasion toward plastics which are usually employed in rotary pressure filter technology in the preparation of methylhydroxyalkylcellulose.
This increased abrasive action of the suspensions obtained in the preparation of methylhydroxyalkylcellulose for sensitive and high-value industrial applications may lead to an increased, unacceptable number of coloured particles in the end product.